Actuators producing mechanical movement of an object in response to the application of electrical power are well known. Among the types of actuators that respond to the application of electrical power to produce mechanical motion are electrothermal actuators. Examples of such actuators are described in U.S. Pat. Nos. 4,759,189, 4,887,429, and 5,203,171, which are incorporated by reference. Within these electrothermal actuators, a closed chamber contains a working fluid. The working fluid is mostly a liquid at ambient temperature and changes phase to become a gas, when heated. That gas phase of the working fluid expands upon continued heating, increasing internal pressure within the chamber. (In the following description, the reference to the working fluid encompasses both of the liquid and gas phases of that fluid, the gas phase expanding upon heating to provide the motive force of the actuator.)
The chamber includes an electrically powered heater that supplies heat to the fluid, in response to an electrical current supplied to the heater. The heat produces the phase change in the working fluid and the pressure increase within the chamber. In response to the increased internal pressure in the chamber, a flexible rolling diaphragm, usually peripherally clamped to the package of the electrothermal actuator, is displaced. The diaphragm displacement pushes a piston that drives a piston rod in a linear direction or rotationally.
When electrical power is removed from the heater and pressure in the chamber decreases, the piston rod retracts or counter-rotates to the original position or orientation. Typically, an electrothermal actuator includes a return spring urging the piston to withdraw the piston rod into the package of the actuator or to return the piston rod to its original orientation, before rotation. The expansion of the working fluid provides a force that counteracts the restoring force of that return spring.
In previously known electrothermal actuators, the piston rod rotates or reciprocates, i.e., extends and retracts. Those actuators do not provide a piston rod for driving an external mechanical device with linear motion and/or rotary motion in response to electrical power applied to the actuator.